1. Field of the Invention
The present invention relates to a non-aqueous inkjet ink.
2. Description of the Related Art
An inkjet recording system is a system in which an inkjet ink with a high degree of fluidity is sprayed from very fine head nozzles in the form of ink particles, and these ink particles then record an image on a printing target positioned opposing the nozzles. This type of system enables high-speed printing to be conducted with minimal noise, and is therefore rapidly becoming widespread.
In terms of the solvent, inks can be broadly classified into aqueous inks and non-aqueous inks. Non-aqueous inks that do not use water as the ink solvent medium, including solvent-based inks that use a volatile solvent as the main constituent and oil-based inks that use a non-volatile solvent as the main constituent, exhibit superior drying properties to aqueous inks, and also exhibit excellent printability.
The coloring material for a non-aqueous ink may use either a dye or a pigment, although using a dye offers the advantage of particularly favorable color production.
In a non-aqueous ink, an oil-soluble dye is used, and when printing is performed onto plain paper, the oil-soluble dye tends to be readily drawn into the interior of the recording paper together with the non-aqueous solvent medium. As a result, the image density tends to fall, and show-through (a phenomenon in which the ink penetrates through to the underside of the printed item) becomes prevalent.
Another example of a non-aqueous ink that uses a dye is a water-in-oil (W/O) emulsion ink in which an aqueous solution containing a dissolved water-soluble dye is emulsified within a non-aqueous solvent. Patent Document 1 (Japanese Patent Laid-Open No. H04-170475) and Patent Document 2 (Japanese Patent Laid-Open No. H04-183762) disclose, as inkjet inks, water-in-oil emulsion inks comprising an oil phase component, a water phase component containing water and an aqueous dye as the main constituents, and an emulsifier. Patent Document 3 (Japanese Patent Laid-Open No. H05-9421) discloses an inkjet ink comprising a core material, which is composed of a colorant dissolved or dispersed within a first solvent medium, emulsified within a second solvent medium that does not dissolve the core material.
W/O emulsion inks exhibit minimal show-through when printed onto plain paper, but if a dye having low solubility in water is used, then achieving a combination of favorable image density and good ink storage stability has proven difficult.
Conventional W/O emulsion inks require an amount of water that is sufficient to completely dissolve the dye within the ink, meaning increasing the dye concentration within the ink requires a concomitant increase in the amount of water. However, as the amount of water within the ink is increased, the emulsion stability may deteriorate, or a W/O emulsion may not be able to be formed at all. In other words, in conventional W/O emulsion inks, the dye content is limited by the amount of water, and if the amount of water is reduced to enable favorable retention of the storage stability of the ink, then the amount of dye is also reduced, making it impossible to improve the image density.
Patent Document 4 (Japanese Patent Laid-Open No. 2002-212466) discloses an oil-based ink for an inkjet that is formed from a mixed liquid comprising an aqueous dye solution and a reverse micelle solution prepared by dissolving a surfactant in a non-polar organic solvent. However, in a similar manner to W/O emulsion inks, an amount of water that is sufficient to completely dissolve the dye within the ink is required, and therefore improving the image density while ensuring favorable retention of the storage stability is very difficult.
Furthermore, even in non-aqueous inkjet inks that employ self-dispersible pigments, in a similar manner to that described above for inks that employ dyes, an amount of water that is sufficient to completely disperse the self-dispersible pigment within the ink is required, meaning improving the image density while ensuring favorable retention of the storage stability is problematic.
As described above, in those cases where a water-soluble or water-dispersible coloring material is used within a non-aqueous inkjet ink, problems remain in terms of the storage stability and the image density.